st: Predicting with time varying covariates

Hi,
I am working on a Survival Analysis model (under the lognormal distribution)
for which I am trying to predict the mean duration while testing for some X (a
policy variable which is binary) that affects it. The "predict" command in the
manual shows me how I can achieve this with time constant variables. But in my
model, the policy variables change over time and I also have demographic
variables that affect the duration which change over time.
How do I predict the mean duration for spells when I have time-varying
covariates?
Thanks,
Deepa.
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